1. Field of the Invention
This invention resides in the field of systems and methods for the analysis of exhaled breath.
2. Description of the Prior Art
Analyses of exhaled breath from human subjects are of value in many applications, including the diagnosis and management of several physiological conditions. A change in the nitric oxide (NO) concentration in the exhaled breath of a person suffering from asthma, for example, can indicate a change in the level of inflammation in the airway of the person, which in turn can indicate an increase in the likelihood of an asthma attack. Another component of exhaled breath whose concentration can be correlated with a physiological abnormality is carbon monoxide, a rise in which can be an early indication of the onset of hemolytic jaundice. A still further example is hydrogen, whose rise can indicate malabsorption of carbohydrate. These gases are typically present in trace amounts, notably at concentrations in the parts per billion (ppb) range, and concentration changes that are still within trace amounts can indicate abnormalities before they can be detected at the parts per million range.
The utility and reliability of an analyzer for exhaled breath are limited by fluctuations in temperature, humidity, and breath flow, any of which can interfere with and influence the analytical result. Interference due to these factors is especially acute when the analyzer is used for measuring trace amounts of the analyte. Thus, any device that seeks to quantify analytes in trace amounts in exhaled breath must minimize the effect of these factors or compensate for their presence.
Among these factors, the one that is the most difficult to control in most cases is the flow rate of the exhaled breath of the subject through the analytical device. A crude method in the prior art for controlling the breath flow rate is a verbal instruction from the clinician performing the analysis to the subject, calling for the subject to breathe out faster or slower to correct deviations from the design rate for the analyzer. This method is limited by the subject's ability to adjust the flow rate, particularly when the subject is a child, and is generally impractical for trace gas analyses over a wide range of concentration. An alternative is to include a variable flow resistance in the analyzer and altering the resistance to correct for the deviations. A device that incorporates this capability is disclosed in Moilanen, E., et al., U.S. Pat. No. 6,733,463 B1, issued May 11, 2004. The device in the Moilanen et al. patent contains a mechanical, electrically controlled throttle that is controlled by a signal from a mass flow meter inside the device. Unfortunately, the Moilanen et al. device is complex, requiring multiple components under feedback control.